Seguro que es el cabezal ? El movimiento que describis es correcto si mi memoria no falla.
Puede ser que "electronicamente" el cabezal esté sonado ?
Te copio y pego (en inglés) algo que encontre :
OPTICAL SENSORS: The 1571 has three infra-red activated sensors. If these
sensors become contaminated with dust or smoke residue, it can cause the
drive to malfunction. The TRACK ZERO DETECTOR is located at the right rear
of the drive head assembly. It is used during disk Formatting, Initialize,
and whenever the drive seeks track zero. If out of adjustment, contaminated,
or defective, it may cause the drive to "chatter" like a 1541 when the
stepper hits the mechanical head stop. Another failure mode allows the head
to go all the way forward, chatter and stay there. The INDEX DETECTOR (just
to the left of the spindle collar) checks the disk for the index hole.
Although not needed on a Commodore GCR drive, this sensor maintains
compatiblity with the MFM standard when the drive is used in that mode (the
index hole conincides with sector 1). The WRITE-PROTECT SENSOR at the left
front side of the drive senses disk change and write-protect status. It
should be checked and/or cleaned if the drive will not format or write to a
disk, but otherwise works normally. Don't forget to check for a write
protect tab that may have fallen off a disk and got stuck at the sensor.
Sensors can be cleaned with compressed air (to remove dust) and/or a
Q-tip moistened (not wet) with alcohol (to remove residue from cigarette
smoke). Do not remove the mounting screws to gain access to the sensors...
you will upset their alignment. The index and write protect sensors are
accessable through the drive door area with the front cover removed from
the drive. The zero stop sensor is a tight fit. Remove about half the
head from a Q-tip and it will work. You'll need to move the head forward
to get to the sensor.
U5 MASTER OSC 251829-01
Drive powers up normally but no read, DRIVE NOT READY error.
U7 HEAD AMP & STEPPER CTRL 251853-01 or M36A2U57
U8 logic 7406
Normal looking startup (LEDs and motors) and computer resets drive but no
access, 74 DRIVE NOT READY error.